Charles Ellicott Commentary Genesis 26:1

"And there was a famine in the land, besides the first famine that was in the days of Abraham. And Isaac went unto Abimelech king of the Philistines, unto Gerar." — Genesis 26:1 (ASV)

Isaac went ... unto Gerar. — Following the stream of Semitic migration (Genesis 12:15), Isaac had originally intended to go to Egypt, but was commanded by God to remain in the land, and by doing so, he received the assurance that he would be confirmed in the inheritance of the promises made to his father.

Isaac was then dwelling at the well Lahai-Roi, and although the exact site of this place is unknown, it lay too far to the south for Isaac to have gone to Gerar on his direct way to Egypt.